In handling technology, objects are moved from one location to another by means of a handling system in order to transfer them, for example, from a feed belt to a processing machine. Such handling systems have both a horizontal movement device and a vertically operating lift system with a holding unit for lifting and lowering an object in the direction of a vertical hoist axis. The lift system comprises a motor unit with a drive shaft, a rotational angle transducer for the drive shaft and a gear assembly with an input shaft, wherein the motor unit drives the input shaft of the gear assembly with its drive shaft.
The specific motor unit generally includes a servomotor whose rotational speed can be regulated in order to permit defined vertical load movements. The rotational angle transducer which is present at the motor or at the drive shaft thereof can detect rotational angles of 0° to 360° in sufficiently small increments. A very precisely defined vertical position of the holding unit and of the load held therein can be derived from the current rotational angle position in conjunction with the known transmission ratio of the gear assembly, which is necessary for precise positioning of the load.
For the determination of the elevation to be successful, at least two preconditions have to be met: as a first precondition vertical calibration has to be performed in advance. For this purpose, the holding unit or the load held therein is moved to a specific reference elevation value with the greatest possible accuracy and the reference elevation value is determined in a specific position by technical measurement means. At the same time, the associated current rotational angle of the motor drive shaft is detected. From this and from the transmission ratio of the entire gear assembly, more precisely from the stroke of the holding unit, which follows from a 360° rotation of the motor drive shaft, a precise determination of the elevation can be carried out later solely by means of the rotational angle interrogation in conjunction with an integral multiple of individual 360° rotations. As a second precondition it is to be ensured that a fixed assignment between the rotational position of the motor and rotational position of the gear assembly input shaft is maintained during the entire handling operation. If this fixed rotational angle assignment is lost, the complex calibration process described above has to be performed again, which is very work-intensive and time-consuming.
When the load is lifted, a torque is introduced which lifts the holding unit in the direction of the vertical hoist axis, via the gear assembly. The weight of the load and the lifting device itself brings about the lowering. This is counteracted by the motor with its torque in a braking fashion. If the load collides with an obstacle during the lowering, the load is abruptly braked as a result. The motor which has until now been braking in the generator mode then suddenly changes its mode of operation and runs as a motor with its motor mass at the maximum rotational speed into the blocked gear assembly. Direct consequences of this are frequent gear assembly damage which requires this component to be replaced. Such collisions occur frequently, in particular, in the case of putting into service, in the setting-up operation, in the case of fault messages or in the case of control faults.
In order to avoid such collision damage, it is known to use overload couplings with which the drive shaft of the motor unit and the input shaft of the gear assembly are connected and which slip above a limiting torque. However, a reasonable configuration of the limiting torque or slipping torque is difficult in the case of use on the vertical hoist axis since, on the one hand, when the load is lifted a high torque has to be transmitted, while, on the other hand, such a high torque is not necessary during lowering. Furthermore, for the case of collision during lowering it is desirable to have the smallest possible limiting torque or slipping torque. It is necessary to make a compromise which is not always satisfactory. However, in particular the slipping of the overload coupling causes the rotational angle assignment between the motor drive shaft and the gear assembly input shaft and therefore the elevation information which is necessary for precise positioning to be lost. After slipping, the complex elevation calibration described above has to be carried out again.